Various evidence has shown that deterioration of memory arises from synaptic dysfunction triggered by soluble Aβ oligomers (see Klein W L, Trends Neurosci. 24: 219-224, 2001; and Selkoe D J, Science 298: 789-791, 2002). Excessive accumulation and deposition of Aβ oligomers may be the trigger for a series of pathological cascades that lead to Alzheimer's disease (AD). Therefore, therapeutic intervention targeting Aβ oligomers may be effective for blocking these cascades. However, findings on core molecules of this amyloid cascade hypothesis which are responsible for neurodegeneration, particularly on neurodegeneration mediated by Aβ oligomers, originate from in vitro experiments (see Hass C et al.: Nature Review 8: 101-12, 2007). This neurodegeneration has not been proven directly in vivo. The greatest defect of previously reported in vivo experiments is that they failed to demonstrate synaptic toxicity of endogenous Aβ oligomers due to the lack of conformation-specific molecular tools (see Lee E B, et al.: J. Biol. Chem. 281: 4292-4299, 2006). There has been known no technique capable of proving the toxicity within the human brain, an aspect which is difficult to demonstrate even in Alzheimer's disease mouse models. Thus, the in vivo neurotoxicity of endogenous Aβ has been often disregarded. It has been unknown why NFT formation and loss of nerve cells precede senile plaque formation in the human entorhinal cortex, and how Aβ oligomers are involved in this mechanism.